This invention relates to cermet varistors, and more particularly to improved varistor compositions utilizable in varistors of both the thick film and bulk types.
Many varistors are currently available which have nonlinear current-voltage characteristics for suppressing transient voltages. The relationship between current and voltage in a varistor is described by the empirical relation: I = (V/C).sup..alpha.
where:
I = current flowing through the varistor PA1 V = voltage across the varistor PA1 C = constant proportional to the varistor PA1 .alpha. = constant &gt; 1, measure of the non-linearity of the varistor.
In general, it is usually desirable that the .alpha. value be of as large a magnitude as possible, since this value represents the extent of nonlinearity of the device.
Resistors and capacitors in a thick film form are readily accessible for utilization in integrated circuits, and, consequently, the need for a varistor of the thick film type has arisen. Thick film varistors which may be employed in integrated circuits have been developed, and U.S. Pat. No. 3,725,836, issued to Wada on Apr. 3, 1973, discloses such a device. The varistor includes a composition having particles of zinc oxide with as an additive either bismuth oxide, lead oxide or barium oxide and a glass frit material which is free of any metallic oxide additives. The varistors with compositions of the above formulations made in accordance with the disclosed method are said to have .alpha. values within the range of 5 to 8.
Bulk type varistors have been used in applications necessitating protection from high energy transients. These varistors commonly include a pair of electrodes applied to a varistor composition containing a bonded mass of semi-conductive particles. In U.S. Pat. No. 3,496,512, granted on Feb. 17, 1970, to Matsuoka, et al., a nonlinear resistor which includes a sintered body of zinc oxide with silver paint electrodes applied to its opposite surfaces is disclosed. And, in other bulk type devices, various additives have been formulated with zinc oxide. See, for example, U.S. Pat. No. 3,632,528 issued on Jan. 4, 1972, to Matsuoka, et. al., for "Lead-Modified Zinc Oxide Voltage Variable Resistor;" U.S. Pat. No. 3,634,337 issued on Jan. 11, 1972, to Matsuoka, et al., for "Barium-Modified Zinc Oxide Voltage Variable Resistor;" U.S. Pat. No. 3,598,763 issued on Aug. 10, 1971, to Matsuoka, et al., for "Manganese-Modified Zinc Oxide Voltage Variable Resistor;" and U.S. Pat. No. 3,699,058 issued on Oct. 17, 1972, to Matsuoka, et al., for "Uranium-Modified Zinc Oxide Voltage Variable Resistor."
Thick film varistors can be made by mixing the zinc oxide composition with the glass material and a liquid vehicle to form a paste, and heating the paste after applying it to an insulating substrate. The heating temperature should be sufficient to evaporate the liquid vehicle and melt the glass material to bond the zinc oxide composition particles whereby a thick film is formed. Electrodes are then applied to the thick film. Alternatively, the paste can be applied to an electrode disposed on a substrate, and heated in the same fashion. Another electrode is then applied to the resulting thick film on its surface opposite the electrode which is in contact with the substrate. The varistor configuration is generally referred to as coplanar, if the electrodes are situated on the same surface of the thick film; and, as parallel plate, if the electrodes are located on opposite surfaces of the thick film.
Bulk varistors are typically manufactured by formulating a zinc oxide composition with a suitable binder system, and pressing the material in a mold. The pressed mass is then fired at a high temperature, electrodes are applied to its opposite surfaces, and leads are connected to the electrodes.
The proper selection of a glass material is crucial in making thick film varistors, since it bonds the zinc oxide semi-conductive particles together upon melting during exposure to high temperatures and imparts adherence properties to the thick film so that it adequately bonds to the substrate (in a coplanar configuration) or to an electrode (in a parallel plate configuration). However, it has been found that the glass material can be of importance in other respects. The thick film varistors developed thus far by the prior art have not displayed a sufficiently high nonlinearity exponent necessitated by many applications. This situation has proliferated despite attempts to increase the .alpha.-exponent by the careful addition of certain metal oxides to the zinc oxide semi-conductive material. It is a discovery of the present invention that this result has occurred due to the migration of the metal oxide additives used from the semi-conductive material during formation of the thick film. Also, the prior art does not disclose a bulk type varistor employing a glass material in the varistor composition. It has also been found that the addition of a proper glass can result in a bulk type varistor having an elevated .alpha.-exponent and provide a more stable device.
Furthermore, the prior art is deficient in that it does not disclose a varistor composition which can be used to form varistors of both the thick film and bulk types having high nonlinearity characteristics. It is believed that this deficiency has resulted from an inability of the known thick film varistor compositions to adequately form a cohesively bonded mass if pressed in a die; and, from the lack of a glass material in the known bulk type varistor compositions which is needed to bond together the semi-conductive particles and bond the thick film to a substrate or electrode. The present invention provides novel, versatile varistor compositions which are used to form varistors of both thick film and bulk types having highly desirable characteristics in accordance with conventional processing techniques.